Mount Unzen

Timeline

1. ca. 500,000 BP

   Volcanic activity begins

   The Unzen volcanic complex enters its explosive early phase, dominated by pyroclastic flows and phreatomagmatic eruptions over hundreds of thousands of years.

2. 701

   Founding as a sacred mountain

   Tradition holds that the monk Gyoki founded the Daijoin Manmyoji temple and Shimensan / Onsen Shrine here, opening the era of Unzen as a Shugendo mountain.

3. 1653

   Unzen Onsen opens

   Kato Zenuemon is credited with opening the Enryaku-yu bath, the first of the Unzen Onsen springs and the start of one of Kyushu's oldest sulphur-spring towns.

4. 1663

   Kanbun eruption

   A lava flow from the Hando-iwa vent on the north-northeast flank of Fugen-dake, and a flood from the Kujuku Island crater, killed more than thirty people.

5. 1792

   Shimabara Taihen Higo Meiwaku

   The collapse of Mount Mayu-yama on 21 May 1792 triggered an Ariake Sea tsunami that killed roughly 15,000 people across Hizen and Higo — Japan's deadliest volcanic disaster.

6. 1934

   Japan's first national park

   Unzen National Park (later Unzen-Amakusa National Park) is gazetted as the first national park designated in Japan.

7. November 1990

   Eruption resumes after 198 years

   On 17 November 1990, twin steam plumes rise from two vents beside a small shrine near the summit, ending nearly two centuries of relative quiet.

8. 3 June 1991

   Tragedy of the great pyroclastic flow

   A pyroclastic flow at 16:08 races down the eastern flank and kills 43 people, including volcanologists Maurice and Katia Krafft and Harry Glicken.

9. August 1992

   Lahar disaster expands

   A large lahar buries homes along the Mizunashi River in Fukae; eleven of them are later preserved in the Mudflow-buried Houses Preservation Park.

10. May 1996

    End of the eruption cycle

    After 9,432 recorded pyroclastic flows over five and a half years, the eruption is declared over and the summit lava dome is named Heisei-shinzan.

11. April 2004

    National Natural Monument designation

    Heisei-shinzan is designated a National Natural Monument and the surrounding summit zone becomes a Special Protection Area of Unzen-Amakusa National Park.

Detailed History

Mount Unzen's eruptive history reaches back roughly half a million years and breaks into two broad phases — an explosive early phase dominated by pyroclastic flows and phreatomagmatic eruptions, followed by a lava-dome-building later phase. Over the last 100,000 years the focus of activity migrated through No-dake, Myoken-dake and finally Fugen-dake, sculpting today's complex of more than 20 overlapping cones. The mountain was historically called Onsen-san (the hot-spring mountain), and tradition holds that the Buddhist monk Gyoki founded the Daijoin Manmyoji temple and the Shimensan / Onsen Shrine here in 701 (Taiho 1), making the range a centre of mountain worship (Shugendo) for more than a millennium. Among the documented eruptions of the Edo period, a December 1663 lava flow on the north-northeast flank of Fugen-dake and a flood from the Kujuku Island crater the following spring killed more than 30 people; far more devastating was the 1792 (Kansei 4) event known as Shimabara Taihen Higo Meiwaku, in which the collapse of Mount Mayu-yama and the resulting Ariake Sea tsunami killed roughly 15,000 people in Hizen and Higo provinces — the deadliest volcanic disaster in Japanese recorded history. In 1934 (Showa 9) the surrounding area became Japan's first national park, then known simply as Unzen National Park and later as Unzen-Amakusa National Park. After 198 years of relative quiet, Mount Unzen reawakened on 17 November 1990 (Heisei 2) with twin steam plumes beside a small shrine near the summit. Eruptive activity escalated through February to May 1991, and on 20 May 1991 a new lava dome was confirmed forming in the Jigoku-ato crater. The catastrophic pyroclastic flow at 16:08 on 3 June 1991 killed 43 people, destroyed 251 homes and caused damages estimated at 230 billion JPY. One further pyroclastic flow on 23 June 1993 killed one more person. The eruption cycle continued until roughly March 1995 and was declared over in May 1996 after a total of 9,432 recorded pyroclastic events. The new lava dome was named Heisei-shinzan that same year, and on 5 April 2004 it was designated a National Natural Monument, with the surrounding summit zone designated a Special Protection Area.

Cultural Significance

Unzen sits at the centre of Japan's very first national park (1934), and Heisei-shinzan itself is a designated National Natural Monument (2004), while the wider Mount Unzen massif is also listed as a Special Place of Scenic Beauty — a rare layering of protections even by Japanese standards. The 43 victims of the 3 June 1991 pyroclastic flow included Maurice and Katia Krafft, the French volcanologists who pioneered close-range volcanic film, and Harry Glicken of the US Geological Survey. Their loss together with that of Japanese researchers, journalists, fire-brigade members and local residents marked an inflection point in international volcanology and in disaster-coverage ethics. The tragedy at the press position known as teiten forced civil-defence authorities and broadcasters worldwide to rethink rules of access to evacuation zones, and it directly influenced the modern Japanese system of volcanic hazard maps and eruption alert levels. The 1792 Shimabara Taihen Higo Meiwaku tsunami, taking roughly 15,000 lives, remains the deadliest volcanic disaster in Japanese history. Layered onto these modern memories are the older identities of Unzen as a sacred Shugendo mountain, a site of Christian persecution in the early Edo period, and one of Japan's oldest sulphur-spring towns from 1653 — a sequence of cultural meanings spanning more than 1,300 years.

Architectural Details

Mount Unzen is not a single peak but a complex of over twenty overlapping volcanic cones on the central Shimabara Peninsula. It rises on the outer ring of the Chijiwa Caldera centred on Tachibana Bay, with magma fed from a chamber tens of kilometres beneath the bay. The principal peaks include Fugen-dake (1,359 m), Heisei-shinzan (1,483 m, the highest point in Nagasaki Prefecture), Kunimi-dake (1,347 m), Myoken-dake (1,333 m), No-dake (1,142 m), Kusenbedake (1,062 m), and Yadake (943 m). The complex was built over roughly 500,000 years in two phases — an explosive early phase of pyroclastic flows and phreatomagmatic eruptions, then a dome-building later phase. Four main dome groups mark the last 150,000 years: No-dake, Myoken-dake, Fugen-dake, and Mayu-yama on the eastern flank. Built infrastructure reflects the 1990-1995 eruption legacy. The Gamadasu Dome (Unzen Volcanic Area Disaster Memorial Hall) houses a full-scale lava-dome model and a pyroclastic-flow simulation. At Mizunashi Honjin roadside station in Fukae, eleven homes buried by the August 1992 mudflow are preserved beneath a vast roof. A ropeway from Nita Pass carries visitors up Myoken-dake for close-range views of off-limits Heisei-shinzan. At around 700 m elevation, Unzen Onsen vents steam and hydrogen sulphide from over thirty fumaroles in the Unzen Jigoku — a sulphur-spring landscape opened in 1653.